Michelle looked at her notes. ‘Mr Parr-Fielding, we have to ask: did you kill her?’

‘Good God, you’re really not quite catching my drift are you? The answer is no. I can’t even kill spiders.’

The two detectives shared a glance, agreeing to wind up the interview and desperate to escape the heat, even if it meant leaving the questions about his criminal record and debts to another time. Michelle said: ‘As well as the swab, I’m going to have to ask you to surrender your phone so we can trace your contacts and movements, and any laptops and computers. We’d also like to see details of your bank statements.’

Now he seemed genuinely angry. ‘Why are you doing all this if you arrested somebody?’

‘We have to follow everything up,’ Claire said. ‘The patrol car with uniformed officers to look after your electronics will be here in five minutes. In the meantime, can I ask if you would be good enough to turn the heating down, or at least open the windows?’

He shrugged. ‘We can continue in my old combi outside, if you like. It’s a bit battered, but it’s nice and cool.’ He rubbed his hands together. ‘Blimey, its decades since I lured two attractive young women in there.’

The frosty stares he got in response prompted a shrug of contrition. ‘Joke?’ he said tentatively. ‘Tell, you what, I will open the windows.’ He got up.

Gillard was looking at a curious video on YouTube, from 2013. Mrs Roy and one of her assistant chefs were in a school hall showing pupils how to make Indian food, as part of a national campaign to teach children about each other’s cuisines. Youngsters of all shapes and sizes, each with pinafore and chef’s hat, were rushing backwards and forwards between tables, carrying bowls, ingredients and utensils, in a race against time. It was clearly hilarious, with Mrs Roy’s male assistant skidding backwards and forwards athletically between two different hobs, his hat always falling off. A large clock mounted on the wall counted down the last few seconds for each team to finish making their onion bhajis. Mrs Roy, who herself was trying to cook at two separate hotplates, managed to serve up what looked to be highly creditable dishes. Her breathless grin to the camera show that she loved what she was doing. The children cheered mightily at the end.

A phone call from Dr Delahaye caught Gillard unawares.

Gillard clicked off the video. ‘David, how are you?’

‘I’ve been better,’ the forensic pathologist replied. ‘Just this morning I received the toxicology test. Mrs Roy does indeed seem to have accumulated a significant amount of thallium in her system. Her kidneys were packed with it, and the urine samples you found in the fridge were also contaminated.’

‘Was she poisoned?’ Gillard asked, sensing the opening of a new avenue in the investigation to compensate for failures over Jason Waddington.

‘Deliberately? It’s hard to say. Her medical records, which I only received this afternoon, show she first presented with hair loss nine months ago. She also reported tingling in her fingers and toes from time to time and occasional loss of feeling in her extremities. Over the same period she also reported listlessness, tiredness and chronic constipation.’ He sighed heavily. ‘I’m afraid this is all rather consistent with thallium. It’s a particularly dangerous toxin because the body confuses it with potassium, which has a vital role in the nervous system. You could call it a very simple nerve agent.’